1. Field of the Invention
The present invention concerns in general terms a method and device for receiving signals by means of an array of antennas. It applies particularly to the reception by a base station of signals transmitted by one or more mobile terminals.
2. Description of Related Art
Beamforming is well known in the field of narrow band antenna processing. It uses an array of antennas, generally linear and uniform (that is to say with a constant spacing between the antennas) and a signal weighting module. In order to form a beam in reception mode, the signals received by the different antennas in the array are weighted by a set of complex coefficients before being added.
If the vector of the signals received by the N antennas in the array is denoted x=(x0,x1, . . . ,xNxe2x88x921)T and the vector of the weighting coefficients (which will be referred to more simply as the weighting vector) is denoted w=(w0,w1, . . . ,wNxe2x88x921)T, the output signal y of the beamformer will be written:
y=wTxxe2x80x83xe2x80x83(1) 
When it is wished to receive the signal transmitted by a given source (for example a mobile terminal), the weighting coefficients wi are determined so that the reception beam points in the direction of this source. In the majority of cases, the direction of arrival (DOA) of the signal is not known and use is made of one of a number of estimation methods available in the state of the art (for example MUSIC, ESPRIT and derivatives thereof).
If a priori knowledge of a reference signal transmitted by the source (or an estimation of a transmitted signal) is available, it is possible to determine the weighting coefficient so as to minimise the root mean square error between the output of the beamformer and the reference signal. Equation (1) represents a spatial filtering operation and the coefficients of the optimum filter can then be obtained by means of the Wiener-Hopf equation:
wT=RdxRxxxe2x88x921xe2x80x83xe2x80x83(2) 
where Rxx is the autocorrelation matrix of the signals received, that is to say Rxx=E(xxH), and Rdx is the matrix (in this case, here, that of a linear form) for correlation of the reference signal d with the received signals, that is to say Rdx=E(dxH). These matrices must be updated when the spatial transfer function varies.
If it is wished to receive the signals transmitted by a plurality of sources S0, . . . SMxe2x88x921 by means of the array of antennas, it suffices to form a plurality M of beams pointing in the respective directions of arrival of the signals transmitted by the different sources. If a priori knowledge of the reference signals d0, . . . ,dMxe2x88x921 transmitted by the different sources is available, it is possible to determine the weighting coefficients of each of the M beamformers by the matrix:
WT=RdxRxxxe2x88x921xe2x80x83xe2x80x83(3) 
where Rxx is the autocorrelation matrix of the signals received, that is to say d=(d0, . . . ,dMxe2x88x921)T is the vector of the reference signals and Rdx is the correlation matrix between the reference signals and the received signals, that is to say Rdx=E(dxH). The respective weighting vectors of the different beamformers are given by the columns of the matrix W.
This technique has in particular been applied in the field of mobile telecommnunications, notably to the CDMA (Code Division Multiple Access) systems where it is known by the acronym CAAAD (Coherent Adaptive Array Diversity). A CAAAD receiver is illustrated schematically in FIG. 1. It comprises an array of antennas 1000, . . . ,100Nxe2x88x921, beamformers 1100, . . . ,110Mxe2x88x921 receiving the N antennas signals and supplying the beam signals y0, . . . ,yMxe2x88x921 to rake receivers 1200, . . . ,120Mxe2x88x921. Each of the M beamformers points towards the direction of arrival of the signal transmitted by mobile terminal. At the output of each beamformer, a RAKE receiver effects an MRC (Maximum Ratio Combining) combination of the signals relating to the different propagation paths between the mobile terminal in question and the array of antennas. It is assumed that all the propagation paths are received by the beamformer.
The CAAAD reception technique is optimum according to a criterion for minimisation of the root mean square error (MMSE or Minimum Mean Square Error) but is difficult to implement. This is because it requires as many beamformers as there are mobile terminals in the cell. In addition, the inversion of the matrix Rxx (or equivalently the resolution of a system of N linear equations with N unknowns) is a complex operation which, in practice, will have to be performed by dedicated circuits. Furthermore, since the beamforming has to be adaptive in order to follow the movement of the terminal, this operation must be performed frequently, which burdens the calculation resources of the receiver.
One reception technique, more robust and of lower performance, consists of forming a large number of fixed beams, for example beams which are angularly equally distributed, and, for each mobile terminal, selecting the one which supplies the signal with the highest power coming from the said terminal. A receiver operating according to this principle is illustrated in FIG. 2. The beamformers 2100, . . . ,210Lxe2x88x921 with L greater than  greater than N, where N is the number of mobile terminals, form L fixed beams from signals received by the antennas 2000, . . . ,200Nxe2x88x921. The beam signals y0, . . . ,yLxe2x88x921 output from the beamformers are transmitted to a plurality of selection modules 2150, . . . , 215Mxe2x88x921, each selection module being associated with a mobile terminal. Decision modules 2160, . . . ,216Mxe2x88x921 supply to each of the selection modules the index of the beam to be selected. A decision module 216i associated with a terminal i∈{0, . . . ,Nxe2x88x921} effects a correlation between the beam signals y0, . . . ,yLxe2x88x921 and a reference signal transmitted by this terminal, chooses the index of the beam supplying the highest energy and transmits it to the selection module 215i. The reference signals of the different terminals must be chosen so as to be orthogonal: for a CDMA system, use would be made of the pilot symbols transmitted over the uplink channel DPCCH (Dedicated Physical Control CHannel). The beam signal selected by the selection modules 2150, . . . ,215Mxe2x88x921 are then supplied to RAKE receivers 2220, . . . ,220Mxe2x88x921 exploiting the reception diversity within the selected fixed beam.
This reception technique does however have many drawbacks. It requires on the one hand the formation of a large number of beams in order to obtain a good angular aiming resolution. Moreover, it functions badly in a micro or pico-cellular environment in which signal propagates along paths which are broadly angularly scattered because of multiple reflections. In this case, the fixed beam selected for a terminal will very often contain only the propagation path with the highest power and the other paths will not be used by the RAKE receiver downstream.
The aim of the invention is to propose a reception method by means of an array of antennas which does not have the aforementioned drawbacks, in particular which allows optimum reception of signals transmitted by a plurality of sources without requiring large calculation resources.
To this end, the invention is defined by a method of receiving a signal transmitted by a source by means of an array of antennas, in which a plurality of beam signals are formed by weighting of the signals received by the different antennas. According to this method each of the said beam signals is correlated with the replica of a reference signal transmitted by the said source and the said beam signals are combined by means of coefficients obtained from the correlation results.
The coefficients can be obtained by complex conjugation of the said correlation results and the beam signals can be normed prior to their combination.
Typically, the coefficients are obtained from a linear function of the previously conjugated correlation results. Advantageously, the coefficients are obtained by the matrix operation xcex3=xcexa9xe2x88x921xcex1* where xcex3=(xcex30, . . . ,xcex3Lxe2x88x921)T is the vector of the said coefficients, xcex1=(xcex10, . . . ,xcex1Lxe2x88x921)T is the vector of the said correlation results, L is the number of beams and xcexa9 is the matrix of size Lxc3x97L whose coefficients are a function of the correlation of the gain functions relating to the different beams.
According to a first variant, the coefficients xcexa9lxe2x80x2l are obtained by xcexa9lxe2x80x2l=glxe2x80x2Hgl where gl and glxe2x80x2 are vectors formed by angular sampling of the gain functions relating to the beams of indices l and lxe2x80x2.
According to a second variant, the coefficients xcexa9lxe2x80x2l are obtained by xcexa9lxe2x80x2l=∫Glxe2x80x2(xcex8)*Gl(xcex8)dxcex8 where Gl(xcex8) and Glxe2x80x2(xcex8) are the gain functions relating to the channel of indices l and lxe2x80x2 respectively and where the summing is effected over the angular scanning range defined by the array.
Advantageously, the signal resulting from the combination of the said beam signals is subjected to a putting back in phase of the components relating to the different propagation paths between the said source and the said array.
The invention is also defined by a method of receiving, by means of an array of antennas, signals transmitted by a plurality of sources, in which a plurality of beam signals are formed by weighting of the signals received by the different antennas and in which, for each source, the said beam signals are correlated with a replica of the reference signal transmitted by the said source and the said beam signals are combined by means of coefficients obtained from the correlation results.
Advantageously, prior to the combination step and for each beam signal, the components relating to the different propagation paths between the said source and the said array are put back in phase.
Typically, for each source Sm, the coefficients are obtained by means of the matrix operation xcex3m=xcexa9xe2x88x921(xcex1m)* where xcex3m=(xcex30m, . . . ,xcex3Lxe2x88x921m)T is the vector of the coefficients for the source Sm, xcex1m=(xcex10m, . . . ,xcex1Lxe2x88x921m)T is the vector of the said correlation results, L is the number of beam signals and xcexa9 is the matrix of size Lxc3x97L whose coefficients are a function of the correlation of the gain functions relating to the different beams.
According to a first variant, the coefficients xcexa9lxe2x80x2l are obtained by xcexa9lxe2x80x2l=glxe2x80x2Hgl where gl and glxe2x80x2 are vectors formed by angular sampling of the gain functions relating to the beams of indices l and lxe2x80x2.
According to a second variant, the coefficients xcexa9lxe2x80x2l are obtained by xcexa9lxe2x80x2l=∫Glxe2x80x2(xcex8)*Gl(xcex8)dxcex8 where Gl(xcex8) and Glxe2x80x2(xcex8) are the gain functions relating to the beams of indices l and lxe2x80x2 respectively and where the adding is effected over the angular scanning range defined by the array.
The invention is also defined by a device for receiving a signal transmitted by a source or a plurality of sources comprising means for implementing the method disclosed above.